Hillary Clinton told CNN: 'I take responsibility. I'm in charge of the of the state department's 60,000-plus people all over the world.' Photograph: Reuters

Leading Republicans have accused Barack Obama of trying to duck responsibility for the attack in Benghazi after the US secretary of state, Hillary Clinton, accepted the blame for security failures over the killing of the American ambassador to Libya and three other officials by an armed militia.

In an apparent attempt to weaken a potential line of attack on Obama by Mitt Romney in Tuesday's crucial debate, Clinton broke away from a tour of Latin America to tell US television networks that responsibility for the circumstances that led to the death of the ambassador, Chris Stevens, stops with her not the White House.

"I take responsibility," Clinton told CNN. "I'm in charge of the state department's 60,000-plus people all over the world and 275 posts. The president and the vice-president wouldn't be knowledgeable about specific decisions that are made by security professionals. They're the ones who weigh all of the threats and the risks and the needs and make a considered decision."

Republicans reacted with scorn and accused the White House of attempting to shirk responsibility for failures exposed by a hearing in Congress last week, including the state department's refusal of requests to strengthen diplomatic security in Libya even following a series of attacks on American, British and other international targets in Benghazi.

Richard Williamson, a Romney foreign policy adviser, said he expected the Romney to use the debate to press the president to "man up and accept responsibility" for failures that led to Stevens' death.

Senator John McCain praised Clinton for "throwing herself under the bus" to shield Obama. But McCain joined two other Republican senators – Lindsey Graham and Kelly Ayotte – in questioning White House claims that it knew nothing of the deteriorating security situation in Benghazi ahead of the killings on September 11 nor the requests for added protection.

They noted that the attack on the US consulate was "preceded by an escalating pattern of attacks this year in Benghazi".

"If the president was truly not aware of this rising threat level in Benghazi, then we have lost confidence in his national security team, whose responsibility it is to keep the president informed," the senators said. "But if the president was aware of these earlier attacks in Benghazi prior to the events of September 11, 2012, then he bears full responsibility for any security failures that occurred.

"The security of Americans serving our nation everywhere in the world is ultimately the job of the commander-in-chief. The buck stops there."

While Clinton moved to shield the president, she also accepted responsibility only up to a point. The secretary of state said specific decisions about protection in Benghazi were made by others.

"The decisions about security assets are made by security professionals," she told Fox News.

Clinton also sought to shift some of the responsibility for inaccurate statements initially made by administration officials about the circumstances of the assault on the Benghazi consulate.

Five days after the attack, the administration sent the US ambassador to the United Nations, Susan Rice, on to television talk shows to say that the assault was part of a broader backlash across the Middle East against an anti-Muslim video. The administration later backtracked, describing the attack as a preplanned terrorist assault that went on for several hours.

Clinton said Rice was merely conveying faulty intelligence.

"As the intelligence community has now said, their assessment over the last month changed. But everyone in the administration was trying to give information to the best of their ability at the time with the caveat that more was likely to be learned and there would be most likely changes," she told Fox News. "The fog of war, the confusion you get in any kind of combat situation, remember this was an attack that went on for hours."

Rice also denied the administration was covering up the true nature of the attack.

"It was purely a function of what was provided to us," she told the Washington Post.

But McCain remained sceptical.

"They're either deceiving the American people or they are so incompetent they don't deserve to serve," he said.

Larry Sabato, director of the University of Virginia's Centre for Politics, said Clinton's series of interviews were clearly an effort to draw fire away from the White House.

"There's three weeks to the election and Obama's struggling. The last thing he needs is to have this placed on this plate so she's doing her party duty. She swinging into action as Bill [Clinton] swung into action for the party convention," he said.

But Sabato said that while Obama will be relieved to be able to defend himself in the debate by pointing to Clinton's statements, there is little evidence that Republican criticism is having an impact on the election.

"This issue is not cutting. It's not having an impact. People seem to be focused squarely on domestic and economic matters in this election, and I just haven't seen any impact," Sabato said. "The Republican base is interested but they've already made a decision."

The greater impact may ultimately be on Clinton if she decides to make a bid for the presidency in 2016.

"It would come back and it would be used by her opponents. I can't imagine it would by any means be the decisive issue. But if she does run, the critical question among the Obama supporters will be: was she loyal? So far the Clintons are the best thing Obama has got going," Sabato said.